On 9/15/06, Vivian McPhail
class Forkable a where fork :: String -> a -> a -> a
... {- instance (Monad m, Forkable (m a), Forkable b) => Forkable (m a -> b) where fork n a1 a2 a = do a' <- a fork n (a1 $ return a') (a2 $ return a') -}
Let's do manual type checking. First, fork :: Forkable a => String -> a -> a -> a So for Forkable (m a -> b) fork :: String -> (m a -> b) -> (m a -> b) -> m a -> b Then fork n a1 a2 a :: b But you define it as fork n a1 a2 a = do {...} So it should be of type Monad t => t a, not just any `b'. Instead, you can define instance (Monad m, Forkable (m b)) => Forkable (m a -> m b) where ... Note that to compile it you also need -fallow-undecidable-instances and -fallow-overlapping-instances. -- Tolik